[patch] Don't disable const merging if an alias of a constant is llvm.used

[Nick Lewycky]

Rafael Espíndola wrote:
> Right now we disable constant merging not only if a constant is
> llvm.used, but also if an alias of a constant is llvm.used. This patch
> fixes that.

+  /// \brief - Like stripPointerCasts, but don't strip GlobalAlias.
+  Value *stripPointerCastsOnly();

I'm fearful that we're going down the route of clang's IgnoreParens, 
IgnoreParenCasts, IgnoreParenLValueCasts, ignoreParenBaseCasts, 
IgnoreParenImpCasts ...

but anyways, please write the comment without reference to the nearby 
functions. Also appending "Only" doesn't seem clear, something more like 
NoFollowAliases sure is verbose but it's clearer.

Similarly, I'd rather we renamed PSK_ZeroIndices to 
PSK_ZeroIndicesAndAliases, and your patch added the new PSK_ZeroIndices.

That aside, LGTM! I'm not going to block on style/naming issues.

Nick